Application Ser. No. 568,233 discloses and claims an in-band signal-to-noise ratio monitor in which the ratio is determined by deriving the log of the output voltage of a narrow bandpass filter which has its center frequency at the same frequency as the center frequency of the signal. The apparatus as disclosed in said application Ser. No. 568,233 does not distinguish in its output between a signal-to-noise ratio above and below unity so that a low or complete lack of signal cannot be distinguished from a satisfactory signal-to-noise ratio.